Vulcan

Vulcan was a hypothetical planet proposed to exist inside Mercury's orbit to account for peculiarities in Mercury's orbit, suggested by its perihelion shift. The planet was never found and the anomalies in Mercury's remained a mystery for seventy years.
 Urbain-Joseph Le Verrier France 1846 Vulcan

Vulcan was a hypothetical planet proposed to exist inside Mercury's orbit to account for peculiarities in Mercury's orbit, suggested by its perihelion shift. The planet was never found and the anomalies in Mercury's remained a mystery for seventy years.